In positioning systems based on satellite positioning, a positioning receiver attempts to receive signals from at least four satellites in order to determine the position of the positioning receiver as well as the time data. An example of such a satellite positioning system is the GPS system (Global Positioning System), comprising a plurality of satellites orbiting the globe according to predetermined orbits. These satellites transmit Ephemeris data, on the basis of which the position of a satellite can be determined at each moment of time, in case the exact time data used in the satellite positioning system is known in the positioning receiver. In the GPS system, the satellites transmit a spread spectrum signal modulated with a code which is individual for each satellite. Thus, the positioning receiver can distinguish signals transmitted by different satellites from each other by using a reference code corresponding to a satellite code generated locally in the positioning receiver.
A problem in such positioning systems based on satellite positioning is often the fact that the signal transmitted by a satellite is strongly attenuated when it arrives at the positioning receiver, wherein it is very difficult to distinguish the signal from background noise. The signal is attenuated e.g. by climatic conditions and obstacles, such as buildings and surrounding ground topography on the path of the signal. It is particularly difficult to perform positioning inside a building, because the building itself strongly attenuates the signal transmitted by satellites and, on the other hand, multipath propagation may be strong, because possibly reflected signals coming for example through a window are not necessarily as attenuated as signals coming straight through the roof. In this case, the receiver may misinterpret the signal propagation time and the satellite position at the moment of transmission of the signal, due to e.g. said lag in the signal propagation time, caused by the multipath propagation
Each operating satellite of the GPS system transmits a so-called L1 signal at the carrier frequency of 1575.42 MHz. This frequency is also indicated with 154f0, where f0=10.23 MHz. Furthermore, the satellites transmit another ranging signal at a carrier frequency of 1227.6 MHz called L2, i.e. 120f0. In the satellite, these signals are modulated with at least one pseudo sequence. This pseudo sequence is different for each satellite. As a result of the modulation, a code-modulated wide-band signal is generated. This modulation technique allows the receiver to distinguish between the signals transmitted by different satellites, although the carrier frequencies used in the transmission are substantially the same. This modulation technique is called code division multiple access (CDMA). In each satellite, for modulating the L1 signal, the pseudo sequence used is e.g. a so-called C/A code (Coarse/Acquisition code), which is a code from the family of the Gold codes. Each GPS satellite transmits a signal by using an individual C/A code. The codes are formed as a modulo-2 sum of two 1023-bit binary sequences. The first binary sequence G1 is formed with the polynomial X10+X3+1, and the second binary sequence G2 is formed by delaying the polynomial X10+X9+X8+X6+X3+X2+1 in such a way that the delay is different for each satellite. This arrangement makes it possible to generate different C/A codes by using identical code generators. The C/A codes are thus binary codes whose chipping rate in the GPS system is 1.023 MHz. The C/A code comprises 1023 chips, wherein the iteration time (epoch) of the code is 1 ms. The carrier of the L1 signal is further modulated by navigation information at a bit rate of 50 bit/s. The navigation information comprises information about the “health”, orbit, time data of the satellite, etc.
To detect the signals of the satellites and to identify the satellites, the receiver must perform acquisition, whereby the receiver searches for the signal of a satellite and attempts to be synchronized to this signal so that the data transmitted with the signal can be received and demodulated.
The positioning receiver must perform the acquisition e.g. when the receiver is turned on and also in a situation in which the receiver has not been capable of receiving the signal of any satellite for a long time. During the use of the positioning receiver, there may be situations, in which the positioning receiver, which has acquired and synchronized to the signal of a satellite, loses the synchronization. The reason for this may be that the positioning receiver is in motion and variations in the environment cause changes in the signal strength. Also indoors, there may be even large variations in the signal strength at different locations in a building. For example, in the vicinity of a window, the signal strength of a satellite can be considerably higher than in the centre part of the building. Also, there may be differences in the signal strength on different floors. In such a situation, the positioning receiver may lose its synchronization for a moment, and the positioning receiver should be able to perform reacquisition of the weakened signal as soon as possible so that the positioning receiver would not have to perform the actual acquisition process again.
The actual acquisition is performed at the stage when the positioning receiver has no information about the correct code phase of the satellite signal to be received. Thus, the positioning receiver must find out the correct code phase from all the possible different code phases, which, in the GPS system, means a total of 1023 possible code phases. Such a situation comes up, for example, in an independently operating positioning receiver which has not been capable of receiving a transmitted signal for some time. Typically, the length of such a blackout which requires acquisition is in the order of one minute or more. Also, if the positioning receiver has been switched off for a longer time, the actual acquisition must be performed.
Reacquisition refers to a situation, in which the positioning receiver knows, at the precision of a few chips, the code phase of the signal to be received. Thus, the search for the code phase can be limited to a few different code phases close to the correct code phase. Such a situation may come up, for example, when an independently operating positioning receiver is not capable of receiving a transmitted signal for a short time, typically for a few seconds. Also, in such a positioning receiver, in which auxiliary data is received from elsewhere than from a transmitter transmitting the signal to be received, the code phase is known at the precision of a few chips on the basis of the auxiliary data. In this case, the positioning receiver can execute a reacquisition process instead of the actual acquisition process. Such auxiliary data can be preferably transmitted via a mobile communication network or another communication method to the positioning receiver.
Almost all known GPS receivers utilize correlation methods for code acquisition and tracking. Also, correlation methods can be used for reacquisition. Reference codes ref(k), i.e. the pseudo random sequences for different satellites are stored or generated locally in the positioning receiver. A received signal is subjected to conversion to an intermediate frequency (down conversion), whereafter the receiver multiplies the received signal with the stored pseudo sequence. The signal obtained as a result of the multiplication is integrated or low-pass filtered. The presence of the satellite signal can be determined on the basis of this filtered or integrated signal. The multiplication is iterated in the receiver so that the phase of the pseudo random sequence stored in the receiver is shifted each time. The correct phase is determined from the correlation result preferably so that when the correlation result is the greatest, the correct phase has been found. Thus, the receiver is correctly synchronized with the received signal. After the code acquisition/reacquisition has been completed, the next steps are frequency tuning and phase locking.
In receivers according to prior art, attempts have been made to improve the reacquisition of a weak signal for example by using a long correlation time to achieve better distinguishability of correlation peaks. In practice, however, this means that the reacquisition time is long in all situations, also when it would not be required by the signal strength. Furthermore, the power consumption is increased as the correlation time is increased. On the other hand, if the reacquisition time is kept short, the positioning receiver does not operate well indoors and in other places where the signal strength is weak. In the design of the positioning receiver of prior art, the time used for reacquisition is thus determined by the weakest signal strength, at which the positioning receiver must be capable of performing reacquisition. Thus, to achieve an optimal solution, a compromise must be made between this time used for reacquisition and the weakest signal strength. 